Programmable devices are a class of general-purpose integrated circuit that can be configured to accomplish a wide variety of applications. Such programmable devices have two basic versions: mask programmable devices, which are programmed only by a manufacturer, and field programmable devices, which are programmable by the end user. In addition, programmable devices can be further categorized as programmable memory devices and programmable logic devices. Programmable memory devices include programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), and electronically erasable programmable read-only memory (EEPROM). Programmable logic devices include programmable logic array (PLA) devices, programmable array logic (PAL) devices, erasable programmable logic devices (EPLD), and programmable gate arrays (PGA).
Field programmable gate arrays (FPGA) have become very popular for telecommunication applications, Internet applications, switching applications, routing applications, et cetera. FPGAs typically include programmable logic fabric and a plurality of input/output blocks. As implemented on an integrated circuit, the input/output blocks are fabricated on the perimeter of the die surrounding the programmable logic fabric. The programmable logic fabric includes a plurality of configurable logic blocks (CLB) and a plurality of interconnections. Each of the configurable logic blocks can be programmed to perform simple logic functions and/or simple Boolean equations. By programming the interconnections, the programmed logic functions of individual configurable logic blocks can be coupled together to implement more complex logic functions and/or Boolean equations.
Because of its programming diversity, field programmable gate arrays offer end users the benefits of custom CMOS VLSI integrated circuits while avoiding the initial cost, design time delay, and inherent risks of application specific integrated circuits (ASIC). While FPGAs have these advantages, there are some disadvantages. For instance, an FPGA-based design to perform a similar function as implemented in an ASIC requires approximately 25 to 50 times more die area than the ASIC. As such, the manufacturing expense of an FPGA is greater than that of an ASIC. In addition, an FPGA requires significantly more printed circuit board space and consumes more power than an equally functional ASIC.
To mitigate some of the disadvantages of FPGAs with respect to ASICs, some FPGA manufacturers are including ASIC like functions on the same substrate as the programmable logic fabric. For example, FPGAs are now commercially available that include Random Access Memory (RAM) blocks and/or multipliers in the programmable logic fabric. As such, the programmable logic fabric does not have to be programmed to perform RAM functions and/or multiplier functions, when such functions are needed. Thus, for these functions, significantly less die area is needed within the FPGA.
While including such fixed logic functions in the programmable logic fabric offers end users greater design flexibility with less die consumption, end users are demanding greater performance and flexibility from FPGAS. In particular, end users would like to see more fixed logic functionality (i.e., ASIC like functionality) embedded within the programmable logic fabric of FPGAs, while retaining the versatility of traditional FPGAs.
Therefore, a need exists for a programmable gate array that includes embedded fixed logic circuits that uses at least a portion of the FPGA to perform a fixed logic function.